Methods of determining the dimensions of underground cavities



METHODS F DETERMNING THE DIMENSIONS 0F UNDERGROUND QAVITIES goriliipany,Inc., New York, NX., a corporation of N ew Filed June 19, 1958, Ser. No.743,215

6 Claims. (Cl. Z50-43.5)

This invention relates to the storage of liquefied` petroleum productsin storage wells formed in salt formations in the earth and, moreparticularly, to methods for determining the vertical cross section of`storage wells and has for an object the provision of a reliable,simplied method of determining the dimensions of an underground storagecavity.

Use is now being made of storage facilities formed in salt formations inthe earth for the large quantity storage of petroleum products. Usually,a plurality of storage wells or cavities are located in the same saltformation a relatively short distance one from the other. As iskwell-known in the art, the size of a storage well or kcavity formed inla salt formation will increase with use over a period'of time.Therefore, it becomes important to know when the enlargements ofproximate wells become excessive so as to result in the merger of thestorage wells beneath the earth and the mixing of the products storedtherein.

The vertical cross section of a storage well is similar to a teardrop,that is, narrow at the top and wide at the bottom. But this only occursunder ideal conditions of uniform solubility of a salt formation. Suchconditions seldom exist and therefore a true configuration of thestorage well is unknown. Accordingly, the horizontal dimensions of thestorage well at various depths, irnportant when trying to avoid mergerof adjacent wells, can no longer be determined by merely measuring thevolume of liquid contained in the well.

Without information as to the size of the Well, further problems arise.In the manufacture of a storage well, a shoe or seal is usually providedaround the pipes extending into the well to create a pressure vessel inorder that the petroleum products, usually liquefied petroleum gas, maybe stored in liquid form in the well. Should too much of the petroleumproduct be withdrawn from the well, the Water will begin to erode theroof of the cavity and by a leaching action of the salt around the shoeor seal may eventually erode away the roof of the cavity. lt will thenbe impossible to maintain pressure within the well and the petroleumproducts will vaporize and be lost to the atmosphere through the porousearth structure above the salt formation.

Accordingly, it is a further object of the present invention to maintainthe vapor-tight integrity of storage wells formed in salt formations.

In accordance with one aspect of the present invention, there isprovided a method whereby the interface between the petroleum product,usually liquefied petroleum gas, and the salt Water may readily bedetermined. More particularly, the method comprehends the generation ofgamma rays and the detection of scattered rays at a point spaced fromthe point of generation of the gamma rays. By moving the points ofgeneration and detection through the storage Well, there will beobserved a pronounced change in the detection rate of the scatteredgamma raysI when the interface is traversed. Y

Employing this aspect ofthe present invention, it is arent O possiblethen to determine, in accordance with another feature of the presentinvention, the vertical cross section of the underground storage well orcavity and changes therein. More particularly, the liquid interface isinitially located in accordance with the previous method and uid inputto the cavity or storage well is metered. The new position of theinterface is determined and from this information it is possible todetermine the horizontal cross section of the storage well as a functionof depth. A plot of the horizontal dimensions with respect to depth willproduce a vertical cross section of the storage well.

For other objects and advantages of the present invention, reference maybe had to the following detailed description taken in conjunction withthe accompanying drawing in which:

Fig. 1 illustrates a storage well arrangement in a salt formation, and

Fig. 2 schematically illustrates a system suitable for practice of thepresent invention.

Referring now to the drawings, and more particularly to Fig. l, there isillustrated a series of storage Wells 1li-15 located in a saltformation. The boundaries of each of the wells 10-15 are illustrated bythe dotted lines 10a-15a. Each of the wells is employed for storage of aliquefied oil product, usually some form of liquefied petroleum gas,hereinafter referred to as LPG. For example, the well 10 may have storedtherein, pentane, while isobutane will be in well 11; normal butane inwell 12; propane in well 13; a raw mixture of pentane, isobutane, normalbutane, and propane in well 14; and normally gaseous olefins in well 15.

The wells 11i-15 may be formed by employing conven-tional well drillingapparatus to bore a hole into the depths of a salt formation.Thereafter, water preferably warm water, is circulated by way of pipes17 and 18 to erode the surrounding walls of salt and form a cavernwithin the salt formation similar to the cavities 10 and 11, Fig. 2.Thereafter, the product to be stored, for example, pentane, will bepumped from a source, not shown, through pipe 16, thence through theouter concentric pipe or casing 17 and into the storage well 10. Thesalt Water is displaced and travels back to the surface by way of theinner concentric pipe 18. The LPG, being of lighter density than thesalt water, rides on top of the water, and being immisible therewith,creates with the Water a well-defined liquid interface 19.

In order to remove the stored pentane, a reverse process is employed,namely, salt water is pumped into the storage well 10 by way of pipe 18,displacing by positive pressure the stored LPG or pentane to the surfacefor distribution over pipe lines for use at a refinery or for directtransmission to a market.

With continued use of the storage facilities, the size of each well willincrease as illustrated by the dotted lines 10c and 11C due to erosionof the wall structure of the storage wells by the salt water. This isexplained by considering the operation required to withdraw thepetroleum product from a well. Salt water from the surface is pumpeddown through pipe 1S and is heated to the temperature of the surroundingsalt formation. With increased heat, the water is able to take more saltinto solution until saturated conditions exist at the temperature in thewell. Now when the petroleum product is added tothe well, this saturatedsalt water is removed and placed in storage at the surface. lf thesaturated salt solution be permitted to decrease in temperature, saltwill be precipitated out; and when the same water is again used to drivethe petroleum product out of storage, the condition of erosionabove-described will be repeated. While some erosion is desirable inorder to obtain an increase in the size of the storage cavity, un-

controlled erosion can prove quite costly. Should the wall structureseparating wells and 11 'be completely eroded, the product of well 10will be mixed with the ,product of Well 11, resulting in considerableexpense to `the operator. For example, storage Wells of t'he typerepresented by wells 10 and 11 are usually large enough to holdapproximately 250,000 barrels of product. Accordingly, should thepentane in well 10 be mixed with the isobutane in well 11, it would benecessary to transfer 500,000 barrels of product to the refinery for theAcostly operation of reseparating the pentane and .iso-

butane. In accordance with the present invention, it is now possible torecord changes in the cross section of the storage wells so that stepscan be taken fora controlled erosion of the wall surfaces of the wellsor for elimination of erosion by suitable measures, such as, raising thetemperature of the salt water and saturating it completely with saltbefore injection into the storage well.

In accordance with the present invention, the position of the interface19 initially is determined. A measured amount of iluid, either water orLPG, is added to the well and causes interface 19 to move to a newposition 19a. The new position of the interface is then determined. Thehorizontal area of the well may be determined at a known depth of thewell from the information comprising the measured amount of fluid andthe distance x which the interface has moved. The information as to wellarea is useful in determining the storage capacity of the Well. If thewell be assumed symmetrical about the pipe 15, the well radius may bedetermined at a given depth in the well. The assumption of symmetry isnot unreasonable since earth strata are usually uniform in a horizontaldirection. By repeating the above steps and from the information therebyobtained, it is possible to plot a vertical cross section of the welland thereby study the extent of well enlargement due to the leachingprocess.

The method of the present invention may be practiced with a systemincluding a source 30 of gamma rays movable by way of a logging tool 31along the length of the pipe 18 and through the storage capacity of thewell 10. The gamma rays emitted by the source 30 are scattered by thesalt water or by the LPG, and these scattered gamma rays are detected bya suitable detecting means 32 supported by and spaced from the source 30in the tool 31. The detector 32 produces a series of pulses proportionalto the number of gamma rays scattered from the surrounding medium,either salt water or LPG; and these pulses are transmitted uphole overthe cable 33 and are applied by way of commutators 34 to an integrator35. The integrator 35, comprised of resistor 36 and capacitor 37,produces a D.C. signal proportional to the gamma rays detected downholeby the detector 32; and this D C. signal is applied to a null-balancerecorder of the strip-chart type 38. The recorder 38 produces a record,a portion 39 of which is illustrated, comprising a trace 40representative of the signals generated downhole by the detector.

The chart is driven as by way of sheave 51 and mechanical connection 52to move the strip-chart 39 proportionally to the movement of the tool.

Since the number of gamma rays reaching the detector is a function ofthe density of the scattering material, the record 39 will indicatewhether the tool is adjacent the LPG or the salt water. And, further,since the counting rate decreases with increasing density of thesurrounding material, the counting rate will be higher as represented bythe portion 41 of trace 40 when the tool is opposite the LPG and loweras represented by the portion e2 of trace 40 when the tool is oppositethe salt water. When the tool traverses the interface 19, there resultsa sharp change in the counting rate as illustrated by the trace 49; andby reference to the length of chart 39, the operator will immediatelyknow the location of the interface 19.

In one embodiment of a system suitable for carrying out the method ofthe present invention, the source of gamma rays was six millicuries ofcobalt-50, and the detector was separated from the source and shieldedtherefrom with lead absorber a distance of 24 inches. The detector 32was a sodium iodide crystal coupled to a photo-multiplier tube. Asuitable logging tool 31 for carrying out the method of the presentinvention may be obtained from Well Reconnaissance Incorporated, ofDallas, Texas.

While the fluid input may be either salt water or LPG product, there isillustrated in the arrangement of Fig. 2 a meter 45 connected to theinput pipe 18 for measuring the amount of water added to the Well. Meter45 may be of the type to integrate the amount of fluid added to thewell. Where the meter is of this type, the crosssectional area may bedetermined'in the following manner. A fixed amount of fluid, forexample, 10,000 barrels, will be added to the well by Way of pipe 18.Prior to the addition of this fluid, the position of the interface 19will have been located and recorded as the trace 40 on the chart 39.Now, after the xed amount of fluid has been added to the well 10, theinterface 19 will move to a position 19a. The new interface 19a Will belocated in the manner above-described and will appear on the chart 39 astrace 40a. The distance x between the traces 40 and 30a will be ameasure of the total displacement X of the interface from position 19 toposition 19a. Knowing the total change in the interface position and theamount of iiuid added to the well, itwill be simple to compute thecross-sectional area of the well 10.

Should the operator discover that the well 10 is getting too large,i.'e., there is a possibility that the contents of Wells 16 and 11 maymerge, steps may then be taken to prevent the further enlargement of thewell 10. This can be accomplished, for example,'by heating water to beadded to the well to well temperature and saturating it with salt sothat further erosion of the walls of the well will not take place.

It is of further importance in the operation of storage facilitiesformed in salt formations that the well be maintained under sealedconditions. For this reason in the formation of such a well a roof ofsalt is always maintained over the cavity and a seal or shoe 22 providedto act as a packer or seal between the casing or outer pipe 17 and thesalt roof of the well. Y

Because of the ever changing sizes of the storage well, it becomesdifficult to maintain a record of the amount of petroleum product instorage. Accordingly, when there is a large demand for the storedproduct, it will be desirable to withdraw as much of the product fromthe well as possible. However, if too much of the product is Withdrawn,the salt water will begin eroding the roof of the cavity or storage wellcausing enlargements as illustrated by the lines 10b which if permittedto continue would break the seal of the well. Thereafter the Well wouldbe useless for future storage of liquefied petroleum products becauseany product added to the well would no longer be under pressure andwould leak through the porous earth formations above the salt formationand to the atmosphere.

In accordance with another aspect of the present invention, it is nowpossible to maintain a record of the position of the interface betweenthe salt water and the stored petroleum product. In such a record theinterface position can be referenced from the earths surface butpreferably, and for reasons of greater accuracy, the reference may befrom the location or position of the shoe or seal 22. From thediscussion above, it will be apparent that a major consideration indeterminingV the position of the interface is-the prevention of leachingof the roof of the well and thereby the maintenance of the seal aroundthe shoe 22. The location of lthe shoe can be determined from ythedrilling record. On the other hand, Ythe location of the shoe may benoted from a log similar to the portion reproduced in Fig. l. The shoe22 is formed of material more dense than the surrounding salt.Accordingly, as the radioactive source and detector are moved past theshoe, a distinctive change in signal level will be observed at arecorded depth on the chart. Knowing the location of the shoe and,therefore, the roo-f of the storage well, it is now possible to withdrawa maximum amount of the stored petroleum product without endangering theseal.

In accordance with that method, the position of the water-oil productinterface is accurately determined, despite changes in storage capacityof the well due to continued erosion of the storage well by unsaturatedsalt water, by moving a source of gamma rays along a path traversing thedepth of the storage well. The gamma rays are directed into the liquidstored in the well and surrounding the source. At a point spaced apredetermined distance from the source, the gamma rays scattered by theliquid are detected and a signal representative of the number ofscattered gamma rays is recorded with respect to the depth of thesource. The position of the interface is noted as above-described upon asharp change in the recorded values of scattered gamma rays. A portionof a typical chart indicating the position of the interface 19 isrepresented by chart 39 and trace 40.

In the practice of the present invention it has been found that thepositions of the LPG-salt water interfaces may be located with anaccuracy of between one or two feet, depending upon the interpretationof the log produced on the record or chart 39 and upon the sharpness ofthe break between the portions 41 and 42 of the trace 40. Such accuracyis adequate for purposes of aiding operators in locating the interfacein order to determine the cross-sectional area of the well to preventmixing of products stored in adjacent wells and to prevent the erodingof the seal around the casing shoes of such wells. Thus, the presentinvention provides a simple, accurate, and inexpensive method ofobtaining information necessary to the proper operation of storagefacilities within salt formations.

What is claimed is:

l. The method of determining as a function of depth the horizontal crosssection of a petroleum product storage well in a salt formation subjectto water erosion in which the liquid petroleum product forms with saltwater liquid in the storage well an interface and in which a casing forthe addition and removal of salt water liquid extends from the surfacetoward the bottom of the well, comprising the steps of moving a sourceof gamma rays through a column of salt water in the casing and thusalong a path substantially traversing the depth of the storage well,directing gamma rays out of said salt water column and through thecasing into the liquid stored in the storage well and surrounding thecasing, at a point a predetermined distance from the source and withinsaid salt water column and inside said casting detecting the gamma raysscattered by the liquid outside the casing, generating a signalrepresentative of the number of scattered gamma rays detected at saidpoint, recording the signal with respect to depth for indication of thelocation of the interface between the salt water outside the casing andthe petroleum product, adding from the surface a known amount of one ofthe liquids to the storage well and removing at the surface a likeamount of the other of the liquids to displace the position of theinterface, and repeating the above steps to record the new position ofthe interface.

2. The method of determining the cross section of a storage well formedin a salt formation in which a liquid petroleum product is stored withsalt water liquid in the storage well and forms therewith an interfaceand in which the position of the interface previously has been locatedand further in which a casing for the addition of and the removal ofsalt water liquid extends from the surface toward the bottom of thewell, comprising the 6 steps of adding from the surface a known amountof one of the liquids to the well While removing at the surface a likeamount of the other liquid from the well to move the interface to a newposition, moving a source of gamma rays through a salt water columnwithin the casing and along a path substantially traversing the depth ofthe well i and directing gamma rays into the liquids surrounding thecasing, detecting at a point spaced a predetermined distance from saidsource and within the salt water column and the casing the gamma raysscattered by liquid surrounding the casing, recording with respect to adepth function the number of gamma rays scattered and detected, andrepeating the above steps to produce a record suitable for thedetermination of a full profile of the cross section of the storagewell.

3. in a liquid petroleum storage well formed in a watersoluble saltstrata in the earth in which salt water is present to the extentnecessary with the petroleum product to iill the storage capacity of thewell thus to form with the liquid petroleum product an interface and inwhich a casing extends from the surface of the earth toward the bottomof the well for the addition of and the removal of salt water liquid,the method of determining the position of the interface despite changesin the storage capacity of the well due to continued erosion of the saltstrata by unsaturated salt water, comprising the steps of moving asource of gamma rays through a saltwater column within the casing andalong a path substantially traversing the depth of the storage Well anddirecting gamma rays beyond the salt water column and through the casinginto the liquid stored in the well and surrounding the casing, at apoint spaced a predetermined distance from the source and within thesalt water column and the casing detecting the gamma rays scattered bythe liquid outside the casing, generating a signal representative of thenumber of scattered gamma rays detected at said point, and recording thesignal for Vindication of the interface.

4. In a liquid petroleum product storage well formed in a water-solublesalt strata in the earth in which salt water is present to the extentnecessary with the petroleum product to fill the storage capacity of thewell thus to form with the petroleum product an interface and in which acasing extends from the surface of the earth toward the bottom of thewell for the addition to the well and the removal from the well of saltwater liquid, the method of determining the position of the interfacedespite changes in storage capacity in the well due to continued erosionof the salt strata by unsaturated salt water, comprising the steps ofmoving a source of gamma rays through a column of salt water within thecasing and along a path substantially traversing the depth of thestorage well and directing gamma rays beyond the column of salt waterand through the casing into the liquid stored in the well andsurrounding the casing, generating and recording a function related tothe position of the source within the casing, at a point spaced apredetermined distance from the source and within the salt water columnand the casing detecting the gamma rays scattered by the liquid outsidethe casing, generating a signal representative of the number ofscattered gamma rays detected at said point, and recording the signalwith respect to said recorded function for indication of the location ofthe interface.

5. rihe method of withdrawing liquefied petroleum gas stored in a wellformed in a salt formation in which the liquefied petroleum gas forms aninterface with salt water liquid in the storage well and in which acasing for the addition of and the removal of salt water liquid extendsfrom the surface toward the bottom of the well, comprising the steps ofmoving a source of gamma rays through a salt water column within thecasing and along a path substantially traversing the depth of the welland directing gamma rays into the liquids stored in the well andsurrounding the casing, recording the position of the source as it ismoved along said path, at a point spaced 'a predetermined distance fromthe source and within the :salt water column and the casingdetectingtlae gamma rays, scattered by liquid surrounding the casing,generating a signal representative of the number of scattered gamma raysdetected at said point, stopping movement of the source upon thegeneration of a signal sharply de- Viatin'g from a previously generatedsignal and indicative of the position of the interface, removing liqueedpetroleum gas from the well, following with the source the change in theposition of the interface, and stopping the Withdrawal of liquefiedpetroleum gas from the well when the interface has attained apredetermined position from the top of the well.

6. The method of removing liqueiied petroleum gas stored in a Wellformed in a water-soluble salt strata in the earth in which salt Wateris present to the extent necessary with the petroleum product to fillthe storage capacity of the well and to form with the liquefied petroleum gas an interface and further in which a casing for the additionof and the removal of saltwater extends from the surface toward thebottom of the Well, comprising the steps of predetermining a positionalong the depth of the casing as a limit of the liqueed petroleum gas tobe withdrawn from the well, positioning a source of gamma rays in a saltWater column within the casing and at said point and directing gammarays into the :of the arrival of the interface at the vicinity of saidsource stopping the removal of the liqueiied petroleum gas from theWell.

References Cited in the le of this patent UNITED STATES PATENTS2,667,583 Herzog Jan. 26, 1954 2,700,734 Egan et al. Jan. 25, 19552,718,143 Pankratz Sept. 20, 1955 McKay et al. Ian. 20, 1959

